It is planned to investigate the mechanisms of postreplication repair and recombinational repair in an in vitro system. The system will use covalent circular DNA from phages G4 and phi X, and protein extracts from E. coli and later Zenopus eggs of calf thymus. These repair processes are thought to depend on the interaction between demaged DNA carrying pyrimidine dimers and postreplication gaps or interstrand cross-links, on the one hand, and undamaged homologous DNA which, in this case, will be covalent circular RF I molecules appropriately labeled. Experiments will test interaction between the damaged and undamaged molecules in crude cell extracts by transfer of radioactivity and cutting of the covalent circular molecules. Research will be concerned with fractionating and concentrating the proteins required for these repair processes and examining various substrates for their effectiveness in initiating them.